The invention relates to the field of electronic reproduction technology, and concerns a method for white balance in scanning devices for point-by-point and line-by-line scanning of image originals using an optoelectronic scan unit. The scanning devices, also called scanners, can be black/white scanners for scanning black-and-white image originals, or color scanners for scanning color image originals.
In a black/white scanner, a black-and-white image original is illuminated image-point-by-image-point by a light source, and the scan light--modulated corresponding to the brightness values of the scanned image points--is converted by means of an optoelectronic converter into an image signal that represents the brightness values between "black" and "white."
In a color scanner, the scan light coming from the color image original is first split into the color portions "red," "green" and "blue" by means of dichrotic filters, and is supplied to the individual color channels, in which the three color portions of the scan light are then converted into the color signals for "red," "green" and "blue" using optoelectronic converters.
The image signals or, respectively, color signals are digitized in A/D converters, and the digital image values, color values thereby obtained are further processed on-line or are intermediately stored before further processing. The signal preparation stages connected downstream from the optoelectronic converters comprise a defined saturation range, whose maximum signal value is designated the white level.
By means of a white balance of a scanner before the beginning of the scanning, the tonal range of the respective image original to be scanned is matched to the defined saturation range of the signal preparation stages, by using the optoelectronic converter to convert the scan light coming from the brightest point of the image original--the white point--into an image signal value that corresponds to the white level. Since the scan light coming from the brightest point of the image original varies from image original to image original, and the sensitivity of the optoelectronic converter is not constant over a longer period of time, in practice a corresponding white balance is carried out before each document scan, which is particularly expensive in color scanners, since there at least three optoelectronic converters are to be balanced.
In DE-A-25 45 961, a method for automatic white balance in black/white scanners and in color scanners is already indicated. In a calibration phase, the scan unit of a black/white scanner is positioned at the respective white point of the image original, and the scan light coming from the white point that is approached is converted into an image signal actual value in the optoelectronic converter. In a control apparatus, the image signal actual value is compared with an image signal target value that corresponds to the defined white level. A control signal modifies the amplification of the optoelectronic converter and/or of a downstream amplifier until the control deviation is zero. The control signal value required for this is stored for the duration of the document scanning that follows the calibration phase. For white balancing in color scanners, the control means is expanded to the three color channels.
The known method has the disadvantage that in the white balance a corresponding white point on the image original to be reproduced must always be approached with the scan unit, which is time-consuming and imprecise, in particular in case of repetitions of the white balance. In addition, it is often the case that there is no bright image point suitable for use as a white point in a color document.
A further method for white balance for black/white scanners and for color scanners is indicated in EP-A-0 281 659, in which the repeated approaching of a white point on an image original to be reproduced with the scan unit is avoided. For this purpose, when white balancing takes place for the first time, a light attenuation factor is determined by means of optoelectronic scanning of the white point. In repetitions of the white balance, the scan light representing the white point is simulated by the attenuated light of the scan light source, without renewed white point scanning in the image original, whereby the light attenuation occurs by means of an iris diaphragm controlled by the determined light attenuation factor.
The known method is based on a color-neutral density simulation, which is not always given in practice, and can thus sometimes lead to unsatisfactory results.
In black/white scanners for scanning opaque documents, it is also already known to use for the white balance not the brightest point of the opaque document, but rather the reference white of a balance strip, which, in a conventional white balance, must be at least as bright as the brightest point of the opaque document. If, for example, the reference white is darker than the brightest point in the opaque document, and the white level is set to the reference white of the balance strip, there results in the later scanning of the brightest point in the opaque document a signal limitation to the white level, and a loss of information results. Since the reference white of the balance strip changes due to yellowing and soiling, balance errors can disadvantageously result. In order to avoid such balance errors, it has been necessary up to now to replace the balance strip from time to time.